Sodium Salicylate: An In-Depth Thermal and Photophysical Study

Sodium salicylate (2-hydroxybenzoate) has been fully characterised by single-crystal X-ray diffraction (SCXRD), thermogravimetric analysis in combination with in operando FTIR spectroscopy and GC-MS, as well as by UV/Vis absorption and photoluminescence spectroscopy backed up by DFT calculations. SCXRD revealed a layered crystal structure composed of ionic sheets formed by Na⁺ -O contacts sandwiched between π-stacked aromatic rings of the salicylate anion oriented perpendicular to the layer plane. Only weak van der Waals interactions hold the individual sheets together. No solid/solid or solid/liquid phase transitions were observed upon heating, but a three-step decomposition was observed, with the first onset at 245 °C corresponding to concomitant release of CO₂ and phenol. The UV/Vis absorption spectra show temperature-dependent absorption bands at around 305 and/or 345 nm, which according to DFT calculations correspond to the absorption of the carboxylate or phenolate proton transfer species, respectively. In solution, indications of the phenolate species are found only in a very apolar solvent (cyclohexane). Because of excited-state relaxation, emission always occurs from the phenolate structure, which explains the large Stokes shift.